The tools or dies for forming, brazing, and the like typically are massive, must be heated along with the workpiece, and must be cooled prior to removing the completed part. The delay caused to heat and to cool the mass of the tools adds substantially to the overall time necessary to fabricate each part. Delays are especially significant when the manufacturing run is low rate where the dies need to be changed after producing only a few parts of each kind.
Attempts have been made to reduce fabrication times by actively cooling the tools after forming the composite part. These attempts have shortened the time necessary to produce a part, but the time for and cost of heating and cooling remain significant contributors to overall fabrication costs. Designing and making tools with active cooling increases their cost.
Boeing described a process for organic matrix forming and consolidation using induction heating in U.S. Pat. No. 5,530,227. There, prepregs were laid up in a flat sheet and were sandwiched between aluminum susceptor facesheets. To ensure an inert atmosphere around the composite during curing and to permit withdrawing volatiles and outgassing from around the composite during the consolidation, we welded the facesheets around their periphery. Such welding unduly impacts the preparation time and the cost for part fabrication. It also ruined the facesheets (i.e., prohibited their reuse). U.S. Pat. No. 5,599,472 described another Boeing technique that readily and reliably sealed the facesheets without the need for welding and permitted reuse of the facesheets in certain circumstances.
An example of a metal forming process using the Boeing induction heating workcell is described in U.S. Pat. No. 5,420,400. The process combines brazing and superplastic forming of metal with a single induction heating cycle. In such a process, Boeing uses a metal pack or retort to contain the multiple sheets in the workpiece in a pressure zone filled with an inert atmosphere. The sheets are welded along their periphery of the retort. The welds are costly to prepare, introduce trimming as a necessary step to recover the completed part, and limit the reuse of the retort sheets since they must be shaved smaller when trimming away the weld to recover the completed part.
In preparing the retort, we often use temporary seals to hold the sheets until the sheets are clamped into the press. We prefer a "C" spring clamp, as described in U.S. Pat. No. 5,599,472. The clamp sandwiches the outer susceptor sheets of the retort and provides a compressive force to hold the retort together temporarily, pressing the sheets against an "O" ring gasket. Such a gasket seats between susceptor sheets in a machined groove or crimp around the periphery of adjacent susceptors. For processing below about 600.degree. F., the gasket is generally silicone rubber. Between about 600.degree. F. and 1300.degree. F., the gasket is copper; above 1300.degree. F., the gasket is stainless steel. The gasket and susceptor sheets abut and form a gas seal via the compressive force of the die set. The "C" clamp permits handling of the retort in and out of the die set. The "C" clamp also provides a current path from the top sheet to the bottom sheet (when the gasket is rubber or stainless steel). The "C" clamp can be omitted when we use a copper gasket, but handling the susceptor sheets is more difficult. The "C" clamp jumper is only required for electrical continuity when the gasket is not an electrical conductor and, then, only on the edges of the retort transverse to the induction coils since the coils induce eddy currents in the susceptor that flow parallel to the coils.
By "forming," we mean shaping the composite or metal and retort in its plastic state. "Forming" may entail superplastic forming, drawing, hot pressing, or some other shaping operation.
The dies or tooling for induction processing are ceramic because a ceramic is not susceptible to induction heating and, preferably, is a thermal insulator. Ceramic tooling is strengthened and reinforced with fiberglass rods or other appropriate reinforcements to permit it to withstand the temperatures and pressures necessary to form, to consolidate, or otherwise to process the composite materials or metals. Ceramic tools cost less to fabricate than metal tools and also generally have less thermal mass than metal tooling. Because the ceramic tooling is not susceptible to induction heating, it is possible to use the ceramic tooling in combination with induction heating elements to heat the retort without significantly heating the tools. The method reduces the time required and energy consumed to fabricate a part.
Most operations require a susceptor in or adjacent to the workpiece to achieve the necessary heating. The susceptor is heated inductively and transfers its heat principally through conduction to the workpiece that is sealed within the susceptor envelope or retort. Metals in the workpiece may themselves be susceptible to induction heating, but the metal workpiece usually needs to be shielded in an inert atmosphere during the high temperature processing to avoid oxidation of the metal. We enclose the workpiece (one or more metal sheets) in a metal retort when using our ceramic tooling induction heating press. Enclosed in the metal retort, the workpiece does not experience the oscillating magnetic field which instead is stopped in the retort sheets, so heating occurs by conduction from the retort to the workpiece.
Induction focuses heating on the retort and workpiece rather than on the entire tool and eliminates wasteful, inefficient heat sinks. Because the ceramic tools in our induction heating workcell do not heat to as high a temperature as the metal tooling of conventional, prior art presses, problems caused by different coefficients of thermal expansion between the tools and the workpiece are reduced.
To consolidate or to form organic matrix composite materials, an organic matrix composite preform is placed adjacent a metal susceptor. The susceptor heats inductively, and in turn, heats the preform. A consolidation and forming pressure is applied to consolidate and, if applicable, to form the preform at its curing temperature.
The retort often includes three susceptor sheets, typically aluminum, an aluminum SPF alloy, or a `smart` alloy, sealed around their periphery to define two pressure zones. The first pressure zone surrounds the workpiece and is evacuated and maintained under vacuum. The second pressure zone is pressurized (i.e., flooded with gas) to help form the composite panel or workpiece. The shared wall of the three layer sandwich acts as a diaphragm in this situation. In the present invention, we use such a retort and control the tooling pressure across the diaphragm to make delicate, brazed parts. The retort is placed in an induction heating press on the forming surfaces of dies having the desired shape of the molded composite part. After the retort and preform are inductively heated to the desired elevated temperature, pressure is applied (while maintaining the vacuum in the pressure zone around the preform) to consolidate the preform against the die into the desired shape of the completed part.
The susceptor sheets, at least on the outside of the retort, might be a `smart` material that has a Curie point at a desired temperature. For example, for consolidating BMI, we might use INVAR36 and for consolidating thermoplastic polyimides, PERMALLOY and KOVAL. The inner diaphragm sheet typically will be aluminum because it does not intereact with the magnetic field and aluminum generally is less expensive and more readily available than the `smart` materials.
Brazing usually is done in a vacuum furnace. This process involves large facilities costs (it requires significant space in a specialized building), high tooling costs, and long cycle times. The use of induction heating reduces facility cost due to reduced cycle time. Many more panels can be brazed in the same amount of time using induction brazing over the standard vacuum furnace. Also, the same tools used for induction brazing can be used to hot form the facesheets. This eliminates the requirement for a separate high cost tool. Finally, better control of the thermal cycle using induction brazing affords better braze quality. Vacuum furnaces with the characteristic long thermal cycles cannot tailor the thermal cycles to avoid detrimental reactions between the brazing alloys and the base materials. The induction heating process with its rapid heat-up and cool down rates and good thermal control at the critical processing temperatures can tailor the thermal cycle to avoid these detrimental reactions.
When induction brazing honeycomb panels, waviness of the panels can occur if the tooling pressure is insufficient to hold the panel in intimate contact with the die. Usually, 15 to 20 psi is sufficient to keep the susceptor and panel on the die surface. If the core is thin or when brazing occurs at high temperatures (1500-1800.degree. F.), 15-20 psi tooling pressure can cause core crush. The present invention controls the pressure using several, adjustable pressure zones in the retort design to hold the retort (i.e., the part and the susceptor) against the die without crushing the core.
Efforts to reduce tooling mass and to speed cooling apply forced gas to the vacuum furnace technology. Forced cooling gas only slightly improves the cycle time, but, unfortunately, raises the cost of an already expensive operation. Active or forced cooling often produces undesirable temperature gradients across the retort because the cooling occurs unevenly.
Our induction heater uses induction heating and reinforced cast ceramic tools with good durability and affordability. Ceramic dies provide precise control of the geometry of the component while eliminating the requirement to heat the entire tool. These characteristics allow for rapid, controlled heating and cooling down rates which lead to a more efficient, affordable, flexible method of forming and brazing of honeycomb panels. The basic tool design consists of an outer box usually constructed of phenolic materials. Copper tubing to form the induction coils and fiberglass reinforcement rods are held in place with this phenolic box before the ceramic is cast. After the ceramic is cast, the phenolic boards compress the dies when nuts on the end of the reinforcement rods are tightened. Compression of the die increases its durability.
The tool may include a metallic susceptor liner that acts as an antenna for the magnetic flux produced by the coil. The susceptor is typically a 0.045 inch thick layer of magnetic alloy which has a Curie point (temperature at which the alloy becomes nonmagnetic) that matches or closely approximates the desired processing temperature.
An oscillating electrical current (typically, 3 kHz) is passed through the coil, and creates an oscillating magnetic field which emanates from the coil through the ceramic. The magnetic field interacts with the susceptor, which is an alloy having high magnetic permeability. Magnetic flux experiences lower resistance inside such materials. The `smart` susceptors when utilized below the Curie temperature tightly house the flux lines generated by the coil. The high density of time varying magnetic flux in the susceptor creates internal voltages which "induce" eddy currents that are constricted to a thin layer because of the influence of the magnetic permeability of the susceptor as shown by:
CURRENT DEPTH is proportional to ##EQU1##
wherein .mu. is permeability, .rho. is resistivity, and f is frequency.
The current heats the susceptor. The heat is trapped inside the ceramic tool because the ceramic is highly insulative. Cooling of the induction coil tubes by coolant flowing within them also limits how much of the die that is heated. Heating is focused on the part, and the mass being heated is far smaller than the traditional approach. A high degree of the magnetic field coupling (efficient/direct energy transfer) occurs between the magnetic field and the susceptor. All these factors contribute to rapidly heat the workpiece, primarily by conduction from the susceptor to the enclosed part.
As the susceptor heats, any location on the susceptor that reaches the Curie point has its permeability decline significantly and effectively becomes nonmagnetic. That location expels magnetic flux. The flux and current will bend through the remaining high permeability magnetic material surrounding the hot spot. The rate of heating in the nonmagnetic `hot spot` will fall because no current flows there and will increase in the remaining areas until all of the susceptor reaches the Curie point. The induction efficiency when all the material is nonmagnetic is much less. The temperature will hover around the Curie point, because, as cooling occurs anywhere in the susceptor, currents are created in the then magnetic areas. The currents reheat the susceptor to the Curie point. Therefore, if the Curie point is matched to the desired processing temperature, a powerful, repeatable, and simple thermal control mechanism is available.
Heat generated by the induced current in the susceptor is held inside the die cavity because the ceramic material is an insulator. A steep thermal gradient develops through the thickness of the die. The coefficient of thermal expansion for our castable ceramic material is low. Therefore, this material easily supports the thermal stresses that result from this steep gradient. Also, the reinforced nature of the die allows for it to withstand any tensile forces produced by the internal pressurization required to form the facesheets during brazing. Reinforcement is required because the ceramic material is weak in tension but strong in compression. The approach, just as in reinforced concrete, is to utilize the material in compression where it has good durability characteristics. The fiberglass reinforcement rods have good tensile strength and are a dielectric (not electrically conductive) material that does not heat inductively. During processing, tensile forces are counteracted by the compressive preload applied to the die. The net force on the ceramic never reaches tensile loads of any significance.